Adhesives that tolerate incorporation of substantial amounts of water or other polar liquids without unacceptable phase separation, loss of tack, or loss of cohesive strength are needed for a variety of applications. Such an adhesive if made electrically conductive would have particular utility in biomedical electrodes. An adhesive that can be made electrochemically reactive and used with galvanically inert graphite conductors to make a bioelectrode that can recover from polarizing overloads is not known.
Many medical adhesives are known. Exemplary classes are polyvinyl ethers and copolymers of hydrophobic water insoluble monomers such as isooctyl acrylate and a small amount of a water soluble monomer such as a short chain .alpha.,.beta.-unsaturated carboxylic acid (e.g. acrylic acid) or an N-vinyl lactam (e.g. N-vinyl-2-pyrrolidone). While those formulations make excellent medical adhesives, incorporation of substantial amounts of ionic or highly polar solutions results in phase separation.
Conductive adhesives have been known for many years. U.S. Pat. No. 4,066,078 (to Berg), U.S. Pat. Nos. 4,273,135 and 4,352,359 (both to Larimore et al.) and U.S. Pat. Nos. 4,524,087, 4,539,996 and 4,554,924 (all to Engel) are representative. Berg discloses two classes of conductive adhesive plasticized with a polyhydric alcohol. One is a polymer or copolymer derived from the polymerization of an ester of an olefinically unsaturated carboxylic acid and a mono- or polyhydric alcohol having a terminal quaternary ammonium group. The second is sulfated cellulose esters. The procedures for making both classes are cumbersome with multiple steps.
The Larimore et al. patents each show three classes of polymers for use in conductive adhesives. U.S. Pat. No. 4,273,135 describes the first class which is comprised of non-ionic water soluble homo- or copolymers of substantially all water soluble monomers. The second and third classes are copolymers of water soluble monomers' and water insoluble monomers with the third class requiring the polymer to be water insoluble and at least 15% of the carboxylate containing monomers are required. While the electrical properties of these adhesives are suitable for some applications, no crosslinking is disclosed. Crosslinking allows for higher amounts of polyhydric alcohol without reducing viscosity below acceptable levels.
Two of the Engel patents (U.S. Pat. Nos. 4,524,087 and 4,539,996) disclose an electrically conductive adhesive formed by an essentially solventless free radical polymerization of an adhesive precursor having a polyhydric alcohol, at least one ionic monomer, a cross linker and an initiator. The ionic monomers listed are salts of .alpha., .beta.-unsaturated carboxylic acids. The third Engle patent (U.S. Pat. No. 4,554,924) discloses a conductive adhesive formed by an essentially solventless free radical polymerization of an adhesive precursor having a polyhydric alcohol, at least one non-ionic monomer, an initiator, a crosslinker, and an ionizable salt present in an amount sufficient to render the composition electrically conductive. One of the examples describes a precursor comprising 115 g N-vinyl-2-pyrrolidone, 0.3 g triethylene glycol-bis-methacrylate, benzildimethylketal (Irgacure .TM. 651, Ciba Geigy), 25.0 g water, 250 g glycerol, 17.1 g potassium chloride, and 36.0 g polyacrylic acid solution (sodium salt in water (50% by weight)). The resultant adhesive has high tack, but is stringy, displaying poor cohesion so that upon removal it leaves a substantial residue.
The preferred Engel adhesives are used in a variety of biomedical electrodes. These adhesives are lightly crosslinked polymers of acrylic acid in glycerol with water and potassium chloride. While they have excellent properties for some applications, optimization of the electrical properties without adversely affecting adhesion properties has not been possible. Experience has shown that increasing the water content of the composition substantially improves the electrical properties of electrodes coated with the adhesive. Unfortunately increasing the water content to optimum levels for electrical performance is found to decrease the initial tack and cohesive strength of the polymer resulting in poor skin adhesion and residue when the electrode is removed. While tackifiers may be used, they have not been found particularly helpful in optimizing the electrical and adhesive properties. Additionally, prolonged storage (in excess of 2 years) results in loss of tack. This is believed to result from esterification crosslinking of polymeric chains.
Another conductive adhesive is disclosed in U.K. Patent Application No. 2,115,431. The adhesive comprises at least one irradiation cross-linked synthetic organic polymer and an adhesive plasticizer. The cross-linked polymer is formed by subjecting a solution or dispersion of at least one uncrosslinked synthetic organic polymer (including one which has repeating units derived from an N-vinyl lactam monomer) in a solubilizing plasticizer to ionization radiation energies of at least the equivalent of 100,000 electron volts (e.g., x-ray, gamma ray, and electron beam irradiation). Those skilled in the art will appreciate that while the use of ionizing irradiation to force chemical reactions can be useful for many applications, the use of ionizing irradiation is not always desirable because of the wide variety of reactive species that can be produced making the process very difficult to control and making the effect of additional constituents very difficult to predict.
Another art involving polymeric matrices that are swelled in water is the hydrogel art. These compositions are covalently crosslinked and are used extensively in contact lenses. Many of these hydrogels are based on poly(N-vinyl-2-pyrrolidone) and have been extensively used in medical applications. Because of the long experience with use of poly(N-vinyl-2-pyrrolidone) in medical applications its safety is well known making it a desirable candidate for biocompatible adhesives. While most hydrogels are not adhesive, EPO Appln. No. 83305770.6 (publication 0107376, 02/05/84) describes a hydrogel which has some tack and is recommended for use as a wound dressing. The hydrogel is prepared by dissolving between 5% and 25% by weight poly(N-vinyl-2-pyrrolidone) in water and crosslinking with ionizing irradiation (1 to 5 Mrads, electron beam). Here again the ionizing radiation process is not desirable.
Polymeric matrices using multifunctional monomers to crosslink N-vinyl lactams are known in the patent literature. None, however, have been found to be adhesive.
U.S. Pat. No. 3,294,765 discloses crosslinked polymeric matrices of N-vinyl lactams crosslinked with 3,3'-ethylidene-bis(N-vinyl-2-pyrrolidone). The patent indicates that polymeric matrices with mechanical properties ranging from thickened solutions to intractable gels are obtained depending on the relative amount of crosslinker used. None are reported to be adhesive.
U.S. Pat. No. 4,536,554 discloses polymeric network mixtures of polymers formed from hydrophilic monomer N-vinyl-2-pyrrolidone and hydrophobic monomer (5-alkylene-m-dioxanyl)acrylic ester. Crosslinkers identified as suitable for N-vinyl-2-pyrrolidone include 3,3'-ethylidene-bis(N-vinyl-2-pyrrolidone) and a variety of other diallyl, dimethallyl, and divinyl multifunctional monomers. As the polymeric networks are used for contact lenses adhesive characteristics would not be desirable.
The polymeric compositions known in the art have not met the need for an N-vinyl lactam based pressure sensitive adhesive that tolerates the incorporation of water and other polar liquids (particularly glycerol). A need continues to exist for an adhesive suitable for use in biomedical electrodes that will continue to function after prolonged storage. A need also exists for an adhesive that presents no biocompatibility problems and can be manufactured with straightforward processing.